The present invention relates to a device for the detection of power at the output of an electronic circuit such as a power amplifier comprising at least one field effect transistor and one matching network at output.
A matching network at output is understood to mean a network designed to improve or match an electrical characteristic of the circuit. It may be an impedance matching network for example, or a harmonics rejection network in a frequency generator or at the output of an amplifier which, as laid down by a standard, must short-circuit at the second harmonic or to be in an open circuit state at the third harmonic (for example, a class F amplifier). More generally, it is quite rare for an electronic circuit not to have, at its output, a network essentially comprising resistors, inductors and capacitors in order to match it to a specification.
The device according to the invention has been designed for applications in the microwave range (one to several tens of GHZ) but it can also be applied to circuits working at lower frequencies.
The object of this device is the detection of an electrical power present at an output terminal of a circuit: the detection signal can be used to activate an electrical signal that was hitherto dormant, or to servocontrol the supply of a transistor or, again, to trigger an alarm signal for example.
The power detection function is well known per se, and its application is very common in stabilized current or voltage supplies. The principle thereof is given in FIG. 1. In an assembly comprising an electronic circuit 1, such as an amplifier, capable of modulating the current delivered between the input terminals "i" and the output terminals "o", this current I is detected by the voltage created at the terminals of a high value resistor 2. The voltage V.sub.det for the detection of the current I is actually taken at the anode of a diode 3. A high-value resistor 5 at the detection output matches the impedance for the measuring apparatus.
In the device according to the invention, the power detection is achieved by means of a field-effect transistor that is integrated with a matching network comprising at least one inductor and one capacitor. This transistor meets a twofold specification: its gate is grounded and the transistor is not biased, so that it behaves either like a capacitor or like a diode, depending on whether or not it detects a current in the circuit. The field-effect transistor, by its equivalent capacitance, participates in the matching network.